Small, portable medical devices are increasingly making life easier for many people suffering from a variety of conditions. Doctors now routinely treat or control a wide variety of medical conditions with implantable medical devices, such as pacemakers and defibrillators for cardiac arrhythmia; cochlear implants for deafness; deep-brain stimulators for movement disorders like Parkinson’s disease and epilepsy, as well as obsessive-compulsive disorder (OCD); and spinal-cord stimulators for the control of pain. Medical device research is ongoing in many areas; for example, retinal implants are now in development for those who have lost their vision due to degenerative eye conditions such as retinitis pigmentosa or macular degeneration.
These devices must be very compact and draw very little power to be practical. For example, pacemakers today are just a few centimeters long. Compare that to the hockey puck-size implantable pacemakers of the late 1950s and early 1960s. Medical-device manufacturers are constantly striving to reduce their devices’ power consumption along with reducing their form factors. Some of today’s implantable devices don’t even need batteries, which lose charge and require additional surgery if they need replacement. Instead, the devices are powered by inductive radio frequency (RF) links to avoid the need for implanted batteries. Even when such devices do have implanted lithium-ion or zinc-air batteries or ultra-capacitors, manufacturers must strive to keep power consumption to a minimum. That’s why characterizing the load current of medical devices is so important.
Characterizing Load Current Can Be Challenging
Accurately measuring the load current of an implantable medical device can be quite a challenge. Although an implantable defibrillator might draw just 0.5 microamps of current in sleep or standby mode, the current draw could jump to 5A or higher during its active mode, such as when delivering a high energy shock to the heart muscle to restore normal rhythm. They also draw a burst of high current when transmitting status and physiological data.
To characterize the power consumption of a medical device like this accurately, designers and production test engineers need instrumentation that can measure and analyze power consumption in all phases of the device’s operation. They need tools that allow them to make fast, precise measurements of low-level currents when the device is in sleep or standby mode and relatively high currents when the device is in active mode.
Implantable medical devices are in standby or sleep mode for most of their operating life and in active mode only for a short time. The active mode load current waveform, therefore, looks like a pulse, with pulse widths on the order of hundreds of microseconds to milliseconds. The ability to measure load currents over a wide dynamic range is also essential to characterize the device when it’s in active operation or transmitting data wirelessly.
To make a good low current measurement during the sleep or standby mode, you need an instrument with at least one-microamp sensitivity. Sometimes, sensitivity down to a tenth of a microamp or even less is required. In addition, to make stable and accurate measurements, you need an instrument that can make high resolution and high accuracy measurements.
To make these measurements, a couple of features are important. One important feature is the ability to make a measurement over a long measurement interval in order to average out electrical noise created in the device and noise from the external environment. Another important feature is filtering to get rid of unwanted noise. A measurement time that extends over a number of AC power line cycles along with filtering can result in a measurement time that’s well over two seconds. Therefore, to obtain a good low current sleep or standby measurement, you give up speed to get measurement accuracy.
At the other end of the spectrum, instrumentation needs are very different. To capture the active-mode load current pulses caused by a defibrillator’s stimulation of the heart muscle, the measurement instrument must find a load current pulse that’s present for only a few hundred microseconds and make a current measurement while the device is still drawing a high load current. The instrument must respond quickly and make a measurement in a very short time.
In this situation, you give up accuracy to get speed. Consequently, this measurement will have a lower resolution than the sleep-mode or standby-mode current measurements.
Figure 1 shows a typical active-mode current pulse. The instrument making this current pulse measurement must be able to respond to an external signal to know when the medical device is transitioning into active mode, and if the load current pulse has some overshoot as shown, delay making the current measurement until the load current is stable. The instrument should also allow flexibility in selecting a measurement time so that the best measurement can be acquired.
Multiple Test Types Usually Demand Multiple Instruments
Because the needs of making accurate low current measurements during sleep and standby modes, and very fast high current measurements during the active mode are so different, you might assume that you’ll need multiple instruments to make these measurements. You could, for example, place a sense resistor in series with the test lead that connects a power supply to the medical device-under-test and measure the voltage across the sense resistor with a DMM.
Choosing the appropriate value for the sense resistor is challenging. A small resistor value adds only a small additional error to the load current measurement, but if the value is too small, the DMM may not be sensitive enough to measure the low sleep mode current, or even the standby mode current, accurately.
Although an oscilloscope is the appropriate instrument to capture the magnitude of the short, active-mode load current pulses, DC measurement instruments offer greater precision when making DC measurements. To make all the necessary measurements, a power source, a DMM, and an oscilloscope might be required.
A source measure unit (SMU) instrument could be another possible solution for this application. They can measure very low currents (down to picoamps or less) accurately; unfortunately, they are not designed to capture short pulses. Also, they are generally low power instruments and so might not have sufficient total power to deliver the peak current necessary to characterize a device that draws a large amount of peak power such as an implantable defibrillator. In addition, because of their extraordinary sensitivity, SMU instruments can be relatively expensive.
What Would A Single Instrument Solution Look Like?
Obviously, most design and test engineers would prefer a solution that’s less complicated to implement than some combination of a DC power supply to provide the source voltage, a sense resistor, a DMM, an oscilloscope, an SMU instrument, and a switching system to tie them all together. However, instrument designers are only beginning to take on the challenge of creating instruments capable of providing the level of power needed to energize an implantable device without sacrificing the ability to measure both very low load currents and much higher active load currents accurately and with high resolution. Such instruments are only now entering the market in the form of power supplies with integrated precision DC measurement capabilities.
To measure very low standby or sleep mode currents, a power supply/measurement instrument must be capable of DMM-quality measurements with up to 6½ digits of resolution. When making high current measurements, it has to capture current pulses as short as hundreds of microseconds. Also, because some medical devices have a power-up load sequence and a power-down sequence, similar to the one shown in Figure 2, the instrument chosen must have the triggering capabilities needed to make multiple, synchronized measurements at each state of the power-up or power-down cycle.
When evaluating the precision measurement power supply options available, if the instrument is going to be used in an automated test system in addition to the designer’s bench, it’s essential that it offer the LAN, USB, or GPIB interfaces and digital inputs and outputs needed to integrate it with other test equipment in a rack. To simplify characterization on the benchtop, look for a power supply with advanced display capabilities, including built-in graphing functions that simplify monitoring the stability of the load current, capturing and displaying a dynamic load current, or viewing a start-up or turn-off load current.